Generally speaking, the problem of heat exchangers becoming coated in scale is of major importance insofar as it has an important impact on the design and operation of plant which includes this type of heat exchanger. This coating in scale is caused by the deposition of solid materials which flow through the heat exchanger and are then deposited in those areas where the velocity of the fluid is slower.
Thus, in the chemical and petrochemical industries as well as in the generation of energy by thermal or nuclear means, fouling in heat exchangers has a direct impact on their performance and hence on generation efficiency. Not only this, the fouling observed in certain types of plants increases the potential risk of overheating under certain conditions and makes it necessary to reduce production in order to maintain adequate safety margins.
In addition, in the agri-foodstuff industry in particular, fouling of heat exchangers is also crucial in as much as it has measurable consequences in terms of health and safety. This is why heat exchangers have to undergo frequent cleaning operations, the cost of which can be measured in terms of the time for which the means of production are idle and in terms of the consumption of detergent products.
Economic studies have demonstrated than the overall cost of fouling phenomena is especially high and that this prevents the entry of plate-type heat exchangers into certain markets where fouling phenomena have to be well controlled.
Attempted improvements have already been suggested in order to reduce fouling of plate-type heat exchangers; these consist in designing the geometry of the heat exchange areas in a specific manner. The physical phenomena which cause fouling are still not fully understood and no really satisfactory solution has yet been proposed in order to control such fouling. Consequently, on an industrial scale, fouling of heat exchangers is managed empirically and cleaning cycles are scheduled on the basis of knowledge acquired through experience. It is apparent that such a technique is not really suitable for using heat exchangers which employ fluids which have varying compositions. Furthermore, when a cleaning intervention is undertaken too late, fouling may be bad enough to involve extensive, tricky operations. Consequently, cleaning interventions are often scheduled more frequently than necessary and this has a negative impact on the plant's production time.
There is therefore a palpable need to measure or at least evaluate the degree of fouling using a method which can be performed in real-time.
Devices for evaluating the fouling of heat exchangers have already been fitted on gas-liquid tubular heat exchangers. Solutions like those described, in particular, in Document U.S. Pat. No. 6,386,272 are based on the principle of heat flow meters, i.e. on measuring temperature differences either side of a wall. Unfortunately, this principle which works for gas-liquid tubular heat exchangers cannot be transferred to plate-type heat exchangers. Temperature measurements using the heat flow meter principle require a thick wall between the two temperature measuring points and this is virtually incompatible with the plate thickness usually used in heat exchangers which are of the order of 0.5 nm approximately. Moreover, the heat flow meter principle is less sensitive in the context of liquid-liquid heat exchange which is, however, the most commonly-encountered type of heat transfer in the case of plate-type heat exchangers.
One object of the invention is therefore to provide a plate-type heat exchanger which has a device for evaluating the extent to which it has become coated in scale and which works regardless of the type of heat transfer used by the heat exchanger (gas-liquid, gas-gas or liquid-liquid)
Another object of the invention is to provide a solution which can be simply adapted to all types of plate-type heat exchangers, whether they are of a new design or with a view to fitting it in existing heat exchangers.